1. Field of the Invention
The present invention relates generally to a method and apparatus for illuminating an image display, and more particularly to an apparatus and method for illuminating a color sequential image display.
2. Description of the Related Art
In color sequential displays, a display screen is used to display a sequence of monochrome frames corresponding to what will be the red, green and blue components of a final monochromatic image. A typical color sequential display may take form in a reflective LCD micro display. The images generated by the display are illuminated in succession by a red, green, and blue light so that the red light illuminates the red monochromatic frame of the final monochromatic image, the green light illuminates the green frame of the final monochrome image, and the blue light illuminates the blue frame of the final image. Components of a subsequent monochromatic image are illuminated in the same fashion. Switching from one image to the next is performed very rapidly so that an observer sees what is effectively a full color image.
The successive illumination of image frames by red, green, and blue light is typically achieved using a white light source and a rotating color wheel; such wheels are prone to mechanical failure. Alternatively, the successive illumination of monochromatic frames of an image by red, green, and blue light may be achieved using a white light source and a solid-state device such as a liquid crystal polarization switch. Unfortunately this alternative technique has a disadvantage. More particularly, the solid-state techniques that employ devices such as liquid crystal polarization switches work only with polarized light. Accordingly, at least half of the light available for illuminating a particular monochromatic frame is immediately lost. A more important problem with the mechanical and solid-state techniques for illuminating color sequential displays is that only a third of the available white light is used for illuminating the red, green and blue monochromatic frames of the image collectively. In other words, at least two thirds of the available white light is unused at any given moment. For example, when the red monochromatic frame of a final image is displayed, only red light is used to illuminate, while the green and blue components of the white light source are filtered out and unused.
The present relates to a device for producing colored light and an image generating apparatus including such a device. The device includes a switchable light-directing apparatus configured to receive light and a first control circuit coupled to the switchable light-directing apparatus. The first control circuit provides control signals to the switchable light-directing apparatus In response to the switchable light-directing apparatus receiving a control signal, the switchable light-directing apparatus directs a first portion of the received light to a first region of a plane. Additionally, the switchable light-directing apparatus directs second and third portions of the received light to second and third regions, respectively, of the plane. The second region is positioned between the first and third regions of the plane.
In one embodiment, the switchable light-directing apparatus comprises a first group of electrically switchable holographic optical elements comprising first, second, and third electrically switchable holographic optical elements each of which is electrically switchable between an active state and an inactive state. Each of the first, second, and third electrically switchable holographic optical elements is configured to diffract light incident thereon when operating in the active state, and each of the first, second, and third electrically switchable holographic optical elements transmits light incident thereon without substantial alteration when operating in the deactive state. In this embodiment, each of the first, second and third electrically switchable holographic optical elements is activated or deactivated by the first control circuit.